I'm no expert in this field and I recommend you do consult a counselor or a psychologist, if possible, but I happened across your message and thought i'd reply.
Dealing with someone who is in a depressive state or a manic state isn't easy. Someone close to me is bipolar, and it's clear that dealing with the disease has been very difficult for her.
If your boyfriend has been diagnosed as bipolar by a medical professional and if his condition doesn't improve, ask him to make an appointment. Contact his parents, siblings, or friends if you think that would help. Try to remain as calm as you can when discussing this with him, as having an emotional discussion may not help.
My advice is general, and please remember this is just one opinion from someone with no medical training:
1. If your boyfriend has by chance ever hurt you physically, it'd be best if you kept your distance. You didn't mention this, and this may not be an issue, but if your boyfriend has ever hurt you, *regardless* of his mental state, you should be safe first. If you live with him, consider contact a women's shelter.
2. If your boyfriend has been prescribed medication but he has stopped taking it on his own initiative, contact his doctor as soon as you can. Going off medication, changing the dose, or switching medications can have undesired side effects.
3. If your boyfriend is not taking medication, see if he would consider visiting a psychology, psychiatrist, counselor, or clinician to discuss this option.
4. If, in the worst case, you and/or he decide it is best to break up, try not to forget that there are a lot of people out there, and in my opinion there's no such thing as one and only one person for you in the world.
5. If by chance you think your boyfriend's depression could deepen and he might become suicidal, consider hospitalization. (i'm not suggesting he is suicidal, but try to assess the situation as well as you can and discuss it with someone else you know who can view the situation somewhat objectively.)
6. Try to get some sleep, exercise a little to help relieve stress, and don't forget to eat a normal meal. This sort of situation is always difficult to deal with emotionally, but you'll get through it.
Briefly put:
1. Consider your health and safety first.
2. Consider your boyfriend's health and safety.
3. Do what's right for both of you.
4. Don't avoid warning signs if the relationship is in trouble. Seek relationship counseling if you can, but stay within whatever budget you may have.